I think you can get a good bit done in 40 minutes.
examples:
-recovery type ride to just clear your head and break up your day
-15 mins. up, 10x30" on/30" off, 15 mins. cool down
-10 mins. WU, 20 hard as you can go, 10 mins. cool down
-40 minutes as hard as you can go
Ideal? no. but I do think that in most cases 1x40 is better for training than 0x40.